In recent years, lithium secondary batteries, nickel hydride batteries and other secondary batteries (storage cells) have been growing in importance as on-board power sources for vehicles and as power sources for personal computers and handheld devices. Lithium secondary batteries, which are lightweight and provide a high energy density, are advantageously used as high-power on-board energy sources for vehicles. In a typical construction, this type of lithium secondary battery has a positive electrode, a negative electrode, and a porous separator between the positive electrode and the negative electrode. The separator prevents short circuits due to contact between the positive electrode and the negative electrode. In addition, by allowing an electrolyte to be impregnated into pores therein, the separator also serves to form ion-conducting paths between both electrodes.
The separators used up until now have been porous resin sheets composed of, for example, polyethylene (PE) or polypropylene (PP). Because such separators are porous, heat shrinkage arises at elevated temperatures. This effect is used to actuate a shutdown function. However, if the degree of thermal shrinkage is large, localized shorting due to film breakage or the like may arise, and shorting may spread further from this point. To prevent heat shrinkage of the separator, the formation of a porous heat-resistant layer on the separator surface has been disclosed (see, for example, Patent Literature 1). The formation of a porous heat-resistant layer at the surface of an electrode (the positive electrode and/or the negative electrode) in order to keep the positive electrode and the negative electrode from coming into direct mutual contact when heat shrinkage of the separator occurs has also been investigated,